No one is born a criminal, but some people are more likely than others to become criminals because of how the genes they are born with interact with their upbringing. In a new study published in the International Journal of Neuropsychopharmacology, researchers found that experiences — both positive and negative — influence how genetic variants affect the brain and therefore behavior.
"Evidence is accumulating to show that the effects of variants of many genes that are common in the population depend on environmental factors. Further, these genetic variants affect each other," explained Sheilagh Hodgins, co-author of the study, in a press release.
"We conducted a study to determine whether juvenile offending was associated with interactions between three common genetic variants and positive and negative experiences."
For the study, the researchers surveyed 1,337 students aged 17 or 18 in Västmanland, a Swedish county. Along with a saliva sample for DNA, participants provided them with anonymous answers to questions on delinquency, family conflict, experiences of sexual abuse, and the quality of their relationship with their parents.
With the collected data, researchers studied the relationship between the Monoamine oxidase A (MAOA) gene, a key enzyme in the catabolism of brain neurotransmitters, monoamines, especially serotonin, and life experiences.
"About 25 per cent of Caucasian men carry the less active variant of MAOA. Among them, those who experience physical abuse in childhood are more likely than those who are not abused to display serious antisocial behaviour from childhood through adulthood," Hodgins said.
"Among females it is the high activity variant of the MAOA gene that interacts with adversity in childhood to increase the likelihood of antisocial behaviour."
They then looked at the brain-derived neurotrophic factor (BDNF) gene which modulates neuronal plasticity.
"The low expressing variants of BDNF are carried by approximately 30 per cent of individuals and some previous studies had shown that this variant was associated with aggressive behaviour if carriers were exposed to aggressive peers," Hodgins said.
Lastly, they studied the serotonin transporter 5-HTTLPR, and found that approximately 20% of individuals carry the low activity version of this gene.
Of the carriers, "those exposed to adversity in childhood are more likely than those who are not to display antisocial and aggressive behaviour," explained Hodgins.
So, in sum, researchers found that variants of three common genes, MAOA, BDNF, and 5-HTTLPR, interacted with each other, along with negative environmental factors (family conflict and sexual abuse), to increase the risk of criminal behavior in teens. But the genes also work in the other direction; with a positive parent-child relationship, they can actually decrease the risk of adolescent delinquency.
These findings add further support to other studies that show that genes make us more vulnerable to our environments, whether they're positive or negative. In other words, nature and nurture are partners that go hand-in-hand — certainly not nemeses that should be constantly pitted against one another.